Electric resistance welded steel pipes are used to drill or transport oil or natural gas. However, because of their low reliability due to low toughness of electric resistance welded portions, the electric resistance welded steel pipes are only used for applications that do not require severe specifications with respect to low-temperature toughness and resistance to hydrogen induced cracking (hereinafter also referred to as HIC)
To address such a problem, for example, Japanese Examined Patent Application Publication No. 01-58253 discloses a method of manufacturing a high-strength electric resistance welded steel pipe having excellent low-temperature toughness that includes performing electric resistance welding of a steel sheet having a predetermined chemical composition, heating the electric resistance welded portion at a temperature in the range of 790° C. to 1050° C. for at least 5 seconds, and rapidly cooling the electric resistance welded portion at a temperature of 770° C. to 890° C. at a cooling rate of 30° C./s to 150° C./s, thereby forming an electric resistance welded portion having a fine acicular ferrite microstructure. However, the steel sheet used in Japanese Examined Patent Application Publication No. 01-58253 has a small thickness of approximately 8.0 mm. Furthermore, the electric resistance welded portion of the steel pipe thus manufactured has a fracture (appearance) transition temperature of approximately −40° C. at most and, therefore, the low-temperature toughness of the electric resistance welded portion is not significantly improved. Japanese Examined Patent Application Publication No. 03-60888 discloses a method of manufacturing a high-strength electric resistance welded steel pipe having excellent low-temperature toughness that includes performing electric resistance welding of a steel sheet having a predetermined chemical composition, heating the electric resistance welded portion at a temperature of 790° C. to 1050° C. for at least 5 seconds, rapidly cooling the electric resistance welded portion at a temperature of 750° C. to 950° C. at a cooling rate of 30° C./s to 150° C./s, thereby forming an electric resistance welded portion having a fine acicular ferrite microstructure, and heating the electric resistance welded portion at a temperature of 400° C. to 700° C. for 1 minute or less to perform a stress relief heat treatment.
Japanese Examined Patent Application Publication No. 07-42509 discloses a method of manufacturing a high-strength electric resistance welded steel pipe having excellent low-temperature toughness that includes performing electric resistance welding of a steel sheet having a predetermined chemical composition, heating the electric resistance welded portion at a temperature of 850° C. to 1000° C., rapidly cooling the electric resistance welded portion from the Ar3 transformation point or higher to a finish cooling temperature in the range of (Ar1 transformation point−50° C.) to (Ar1 transformation point−100° C.) at a cooling rate of more than 30° C./s to 100° C./s, and performing weak cooling of the electric resistance welded portion.
However, application of the method disclosed in Japanese Examined Patent Application Publication No. 03-60888 to a thick-walled electric resistance welded steel pipe having a thickness of more than 20 mm requires a huge furnace for stress relief (stress removal) heat treatment, thus causing a economical problem. In the application of the method disclosed in Japanese Examined Patent Application Publication No. 07-42509 to a thick-walled electric resistance welded steel pipe having a thickness of more than 20 mm, it is difficult to heat the thick-walled electric resistance welded steel pipe from the outer surface thereof so that the central portion temperature in the direction of thickness of pipe is 850° C. to 1000° C.
It could therefore be helpful to provide a high-strength thick-walled electric resistance welded steel pipe having not only excellent low-temperature toughness but also excellent HIC resistance. The term “excellent low-temperature toughness”, as used herein, means that both the base steel portion and the electric resistance welded portion have a circumferential absorbed energy vE−50 of 150 J or more at a test temperature of −50° C. in a Charpy impact test in accordance with JIS Z 2242. The term “excellent HIC resistance”, as used herein, means that both the base steel portion and the electric resistance welded portion have a crack area ratio CAR of 5% or less after immersion in a NACE Solution A (0.5% CH3COOH+5% NaCl+saturated H2S) specified in NACE TM0284.